This invention relates to methods for fabricating gradient-index rods and rod arrays, particularly gradient-index glass lenses with a radial index profile.
The radial gradient refractive index (xe2x80x9cGRINxe2x80x9d) glass rod is one of the fundamental optical communication system components. It is widely employed, for example, as an optical beam collimator in optical communication devices to couple signals and pump diode lasers into single mode optic fiber, to convert a diverging beam from a fiber into a collimated beam, and to refocus a collimated optical beam into an optical fiber. I. Kitano, H. Ueno, M. Toyama, xe2x80x9cGradient-index lens for low-loss coupling of a laser diode to single-mode fiber,xe2x80x9d Applied Optics, Vol. 25(19), pp.3336, 1986. Most fiber optic devices, from isolators to nxc3x97n switches and dense wavelength division multiplexing devices, depend on a collimated optical beam to achieve minimum insertion loss and maximum efficiency. T. Towe, S. Cai, xe2x80x9cOEM optical components: Gradient-index lenses make light work for beam directing,xe2x80x9d Laser Focus World, Vol. 35, No. 10, 1999. Most necessary processing for the optical signals are carried out in the physical space between the collimators.
Many techniques have been investigated to fabricate radial GRIN materials including ion-exchange, I. Kitano, K. Koizume, H. Matsumura, T. Uchida, M. Furukawa, xe2x80x9cA light-focusing fiber guide prepared by ion-exchanged techniques,xe2x80x9d J. of Japan Society of Applied Physics, Vol. 39, pp.63, 1970, H. Kita, I. Kitano, T. Uchida, M. Furukawa, xe2x80x9cLight-focusing glass fibers and rods,xe2x80x9d J. Am. Ceram. Soc., Vol. 54, pp.321, 1971, S. Houde-Walter, xe2x80x9cRecent progress in gradient-index optics,xe2x80x9d SPIE Vol. 935, 1988; sol-gel, M. Yamane, J. B. Caldwell, D. T. Moore, xe2x80x9cPreparation of gradient-index glass rods by the Sol-Gel process,xe2x80x9d J. Non-Cryst. Sol., V.85, pp.244 (1986), K. Shingyouchi, S. Konishi, K. Susa, I. Matsuyama, xe2x80x9cRadial gradient refractive index glass rods prepared by a sol-gel method,xe2x80x9d Elec. Lett., V.22 pp.99, (1986); molecular stuffing, J. H. Simmons, R. K. Mohr, D. C. Tran, P. B. Macedo and J. A. Litovitz, xe2x80x9cOptical properties of waveguide made by a porous glass process,xe2x80x9d Applied Optics, 18, pp.2732, (1979); diffusion in plastics, D. Hamblen, Kodak and U.S. Pat. No. 4,022,855; chemical vapor deposition, M. A. Pickering, R. L. Taylor, and D. T. Moore, xe2x80x9cGradient infrared optical material prepared by a chemical vapor deposition process,xe2x80x9d Appl. Optics, 25, pp.3364, 1986; photochemical, N. F. Borrelli and D. L. Morse, xe2x80x9cPlanar gradient-index structures,xe2x80x9d in Technical Digest, Topical Meetings on Gradient-index Optical Imaging Systems, Kobe, Japan, D1, 1983; as well as neutron irradiation, P. Sinai, Applied Optics, 10, 99, 1971. Commercial radial GRIN rods are mostly fabricated by ion-exchange and sol-gel.
Ion-exchanged cesium or thallium glasses have been widely used for fabricating radial GRIN rods. Cesium (Cs+) or thallium (Tl+) glasses have many advantages, including excellent optical transparency, but they suffer from many serious drawbacks. The first is the very high toxicity of Cs+ or Tl+ ions in the mother glass rod. This is particularly dangerous in the glass melting stage where the temperature is high and the vapor pressures are high. The second is that the selection of the refractive index of the mother glasses is restricted. The refractive indices of the mother glasses are high. A high refractive index will produce a high back reflection. The third is the slow ionic diffusion process. Typically, the glass rod has to be dipped in the molten salt at a temperature around 500xc2x0 C. for several hundred hours. The maximum diameter of the radial GRIN rod is limited to 3 mm due to the low ionic diffusion process.
The silver ion is sometimes used as an alternative to the thallium ion because it has a higher mobility, S. Houde-Walter, D. T. Moore, xe2x80x9cDelta-n control on GRIN glass by additives in AgCl diffusion baths,xe2x80x9d Applied Optics, Vol. 25(19), pp.3373, 1986. However, it is difficult to fabricate silver glasses because the silver tends to precipitate out of the glass or form a fine metallic colloid in the glass, R. H. Doremus, xe2x80x9cOptical properties of small silver particles,xe2x80x9d J. Chem. Phys., V.41, pp.414, 1965. A technique called xe2x80x9cion-stuffingxe2x80x9d avoids the difficulty of making silver glass from a batch melt, S. Ohmi, et al, xe2x80x9cGradient-index lenses made by double ion exchange process,xe2x80x9d Appl. Opt., V. 27, pp.496, 1988. The technique uses a sodium containing glass and exchanges the sodium for silver in a silver nitrate molten salt. The second step is to immerse the glass rod in a sodium nitrate molten salt and exchange some of the silver ions out near the surface of the glass. So the glass rod has a higher silver concentration at the center than the surface. The major problem for radial GRIN rods fabricated by this process is the photostability. The glass will be colored due to the metal colloid.
The sol-gel method involves the synthesis of a multi-components alkoxide gel, which is shaped by a mold. Since the gel is porous, dopants can be removed or introduced rapidly by immersing the gel in acids for leaching or in a solution containing dopants for introducing. After the diffusion, the gel is dried and sintered to yield a GRIN glass. The sol-gel method has two major advantages over the ion-exchange technique. The first is the rapid transport process. Since the diffusion of leaching is conducted in a porous gel, the transport process is much more rapid than the ionic diffusion process in ion-exchange technique. It means that a larger diameter radial GRIN rod can be fabricated. The second is that a variety of ions can migrate through the gel pores. Bivalent ions can be easily transported through the gel pores. Here, major drawbacks for sol-gel method are that it is difficult to control the index profile and fracture during sintering. Since ions will continue to migrate during the drying and sintering processes, the index profile is difficult to control. During the drying and sintering processes, the outside part consolidates more quickly than the insider part. Thus, gaseous by-products can be trapped in the middle of the material. The thermal expansion coefficient of the inside part will then be higher than the outside part, which causes fracture during sintering process.
There is another type of GRIN material, the axial GRIN material, in which the refractive index varies along optical axis, D. T. Moore, xe2x80x9cDesign of singlets with continuously varying indices of refraction,xe2x80x9d Journal of the Optical Society of America, Vol. 46(7), pp.998, 1971. In one known technique for fabricating axial GRIN glass, disclosed in U.S. Pat. Nos. 5,630,857 and 5,689,374, glasses having different refractive indices are stacked together. These glasses are then thermally diffused at a high temperature to form a monolithic glass blank with a smooth refractive index profile. However, the end surfaces of an axial GRIN must be individually polished to spherical shape in order to focus the optical beam, M. Murty, xe2x80x9cLaminated lens,xe2x80x9d Journal of the Optical Society of America, Vol. 61(7), pp. 886, 1971, which limits their application. By comparison, radial GRIN material has inherent focusing power since the refractive index varies perpendicularly to optical axis. The end surfaces of radial GRIN rods are flat, which simplifies the fabrication process and increases the compatibility significantly.
Therefore, there is a need for an improved method for forming GRIN optical elements that is safe and efficient, produces clear glass, and produces radial GRIN rods directly.
The present invention solves the aforementioned problems and meets the aforementioned need by providing a novel method of fabricating a GRIN optical element. In accordance with the invention, a central rod of optical glass having predetermined properties and predetermined outside dimensions is placed inside a tube of optical glass having predetermined properties and predetermined outside dimensions, to form an assembly. The inside dimensions of the tube are substantially equal to the outside dimensions of the rod. The tube is formed of a plurality of coaxial sleeves, the outside dimensions of each interior sleeve being substantially equal to the inside dimensions of the next adjacent sleeve, each sleeve having a selected refractive index so that, together with the central rod, the refractive indices from the central rod through the outermost sleeve approximate the refractive index profile of the GRIN element to be fabricated. The central rod and sleeve material are selected so that their respective thermal indices of expansion are substantially equal.
The rod and tube assembly is placed in a mold having inside dimensions substantially equal to the outside dimensions of the tube. The mold is oriented vertically so as to ensure that there is no net lateral gravitational force on the assembly. The mold is then heated with the assembly therein at a predetermined temperature for a predetermined time so as to cause a selected amount of diffusion of material between the central rod and the sleeves and thereby produce a radial refractive index gradient. The temperature is then gradually reduced to anneal the glass. The cooled glass comprises a preform which is then drawn into an elongate, continuous GRIN rod. The continuous GRIN rod is then cut into discrete GRIN rod elements.
GRIN elements fabricated in accordance with the invention may be placed into a glass substrate and drawn into elements that have a desired external shape while retaining the optical properties of a radially symmetric, cylindrical GRIN rod. Arrays of GRIN preforms fabricated using the invention may also be bonded together or imbedded in a substrate, and thereafter drawn to produce GRIN arrays.
Accordingly, it is a principal object of the present invention to provide a novel and improved method for fabricating radial GRIN optical elements.
It is another object of the present invention to provide a method for fabricating radial GRIN optical elements with high fabrication productivity.
It is a further object of the present invention to provide a method for fabricating radial GRIN optical elements with high reproducibility.
It is yet another object of the present invention to provide a method for fabricating radial GRIN optical elements that is flexible in producing elements with various index profiles.
It is yet a further object of the invention to provide a method for fabricating radial GRIN optical elements having a desired outside shape while retaining the optical properties of a radially symmetric, cylindrical gradient-index element.
It is an additional object of the present invention to provide a method for fabricating radial GRIN optical element arrays.
The foregoing and other objects, features, and advantages of the invention will be more readily understood upon consideration of the following detailed description of the invention, taken in conjunction with the accompanying drawings.